Method for manufacturing coated paper and a coated paper

ABSTRACT

A method and apparatus for manufacturing paper, in which method base paper manufactured in a paper machine is calendered and coated to improve its printing properties. Before calendering, the moisture content of the manufactured base paper is brought to 4-14% of the total weight of the paper, after which the base paper web ( 2 ) is taken to a multi-nip calender ( 3 ), the base paper being calendered by the multi-nip calender ( 3 ) and the calendered base paper ( 2 ) coated on at least one side with a layer of coating.

PRIORITY CLAIM

This is a national stage of PCT application No. PCT/FI00/00191, filed onMar. 10, 2000. Priority is claimed on that application, and on patentapplication No. 990558 filed in Finland on Mar. 12, 1999.

FIELD OF THE INVENTION

The present invention relates to a method and apparatus formanufacturing calendered and coated paper. The base paper is calenderedusing a multi-nip calender, for example, a super calender, an OptiLoadcalender, or a Janus Concept calender.

BACKGROUND OF THE INVENTION

The invention concerns the manufacture of high-quality printing papers,using on-line calendering and coating. In on-line solutions, the coatingstations and calender are located on the same line at the end of thepaper machine, the paper web manufactured being led directly from thepaper machine to finishing, without intermediate winding. Calenderedpaper grades are manufactured using off-line equipment, so that two orthree separate calenders, each with its own unwinder and reel-up, areused with a single paper machine. The speed of known multi-nip calendershas prevented their use as on-line calenders with high-speed papermachines. However, new multi-nip calenders and coating stations havebeen developed, which run at speeds that can be raised to match those ofpaper machines, allowing them to be connected directly to the sameproduction line as a paper machine. All multi-nip calenders have severalnips, which usually comprise hard and soft rolls. The surface of thesoft rolls is made from paper or some other suitable fibrous material,or, to an increasing extent nowadays, from a polymer material developedfor this purpose. The hard rolls are generally manufactured from castiron and occasionally from steel, and can usually be heated using oil,steam, or in some other way, for instance, by using induction heating.

Calendering is intended to improve the gloss, smoothness, and otherproperties of the surface of the paper relating to the printability ofthe paper. These properties affect the final print quality.

Smoothness for the print impression is created by subjecting the paperfibres simultaneously to heat and high pressure, by heating hard rollsand pressing the rolls against against each other with great force, tocreate a high delay pressure in the nip between them. Due to theseforces, the paper fibres reach their glass transition temperature, sothat the deformations due to the nip load become permanent. The slippingof the paper against the surfaces of the rolls may also increase thedeformations in the paper and the smoothing effect.

When multi-nip calendering is used, the paper is usually manufactured ina paper machine and, if necessary, coated. In both cases, the uncoatedor coated paper is usually wound onto a batch roll and calendered in aseparate calender. The paper is dried to become extremely dry, with amoisture content of about 1-3% of the total weight of the paper, thepaper then being re-wet before calendering to a relatively high moisturecontent of about 6-10%. The purpose of the drying to a low moisturecontent and re-wetting is to even the cross-direction (CD) moistureprofile. This method is used especially in the manufacture ofsuper-calendered, i.e. SC paper. The short period of storage on thebatch roll evens variations in moisture, in the same way as re-wetting.In present-day on-line calendering, the paper is dried to a very lowmoisture content and is re-wet to a suitable moisture content forcalendering, immediately before calendering. The method is thus nearlyidentical to that used in off-line calendering, except that the storagethat evens the moisture content is not used.

Re-wetting can be carried out using, for example, the waterjet unitsdisclosed in U.S. Pat. No. 5,286,348, which create a good moistureprofile in the cross direction of the web.

Problems arising from drying and re-wetting include the time requiredfor the moisture content to even out and the increased energy requiredto evaporate the water needed for wetting. The greater dryingrequirement increases the length of the machine and the space needed forit, compared to equipment in which wetting is not required. An unevenmoisture content will lead to variations in smoothness and in thethickness profile, because the moisture content has a great effect onthe deformation of the fibres. If the thickness profile is uneven,winding becomes difficult and transverse buckling may even occur incustomer reels. This buckling reduces the paper's runnability in thepreparing and printing machines and thus reduces the quality of the endproduct in the eyes of the customer.

Nowadays, the moisture profile of paper during manufacture is controlledin several ways, especially at the start of the formation of the web. Inpresent processes, the control of the moisture profile is intended toensure good runnability of the machine and the product beingmanufactured. This is understandable, because the tension profile, whichgreatly affects runnability, depends greatly on the moisture profile. Inoff-line calendering, the aim is to keep the moisture profile as even aspossible, in those parts of the process in which it has the greatesteffect on runnability.

A reason for coating paper is to improve its printability. Coating isused to influence the whiteness of the paper, the evenness of itssurface, and the gloss of the print surface. Thus, coating has partlythe same objectives as calendering. How thick a coating layer is used,what coating mixes are suitable, and how many times the paper is coatedall depend on the use of the paper. The coating mix can be spread ontothe surface of the paper in many different ways, each one of whichcreates a different final result, and surface on the end product.Usually, thicker coating and more coating layers will lead to a betterprint surface, so that, in art-paper grades, there may be severalcoating layers and a total coating amount of several tens of grams persquare metre on each side of the paper. In lightweight coated printing(LWC) papers, the amount of coating on one side of the paper issignificantly smaller, usually about 5-15 g/m². Smaller amounts ofcoating do not necessarily cover the entire surface of the paper,instead the coating remains in the valleys formed in the surfaceroughness of the paper by the action of the doctor blade or otherevening device. Thus, the coating thickness varies according to theroughness profile of the paper, so that the properties of the printsurface are not absolutely even. However, the printing properties of LWCpaper are better than those of corresponding uncoated paper, so that itscheaper price makes it quite suitable for uses requiring a reasonablygood print surface at a low price. Naturally, the properties of LWCpaper depend decisively on the type and quality of the base paper, andon the amount and type of coating used. Because a small amount ofcoating will not greatly increase whiteness, the base paper itself mustbe sufficiently white. Thus, LWC base paper usually contains chemicalpulp fibres, which are intended not only to increase strength, but alsoto improve whiteness. Due to its characteristics, LWC paper is notusually calendered, as calendering cannot substantially influence theproperties of this paper grade.

Paper based on mechanical pulp containing a great deal of filler canonly be coated, if the filler content is sufficiently low. Base papermanufactured from mechanical pulp is weaker than paper made fromchemical wood-free pulp and the addition of fillers further reduces itsstrength. For example, even under favourable conditions, the maximumfiller content that can be used in blade coating is 15%. Thus, paper atleast partly based on mechanical pulp and containing a large amount offiller has not been manufactured.

Another problem relating to coated papers, and particularly thosecontaining filler, is recyclability. If the paper is recycled for fibre,the filler and coating must be removed, which demands a great deal ofenergy and especially strong chemicals. If this type of paper is finallyused as a fuel, the coating and fillers remain as unburned ash, whichhinders combustion and which is difficult to exploit economically. Theamount of ash is also affected by the quality of fibre in the basepaper. Though pulp will burn almost completely, groundwood fibrescontain incombustible substances, which increase the amount of ash.

SUMMARY OF THE INVENTION

The invention is intended to create an entirely new type of method, bymeans of which it is possible to manufacture a new type of paper, andachieve, at lower cost, at least the same quality of print surface as inpresent paper grades intended for corresponding uses, especially LWCpaper grades.

The invention is based on the calendering the manufactured paper web ina multi-nip calender while wet and then coating the calendered paperwith a very small amount of coating. The resulting paper preferably hasa layer of coating on at least one side with a maximum grammage of 5g/m².

Considerable advantages are gained with the aid of the invention.

The invention can be used to produce paper of a quality corresponding toLWC paper, with lower production costs, by very lightly coating a basepaper produced from groundwood fibre. The invention can be used tomanufacture coated paper from a base paper of low basic strength,because calendering carried out according to the invention increases thestrength of paper. By optimizing the process, it is even possible toachieve a higher quality. The method can be used to reduce the price ofhigh-quality lightweight-coated printing paper to an entirely new level.Because the total amount of coat is reduced, the amount of substance tobe separated in recycling is reduced, compared to coated paper made forma corresponding base paper. Because the amount of coating is small, thegrammage of the base paper can be correspondingly greater, allowing theuse of fibres providing a lower strength, or a large amount of filler,without decreasing the paper's strength or runnability. If it ispossible to use groundwood fibre and a large amount of filler instead ofpulp fibre, the price of the base paper will remain low, while, whenusing thin coating layers, the price of the coating will also be low.Because only a small amount of coating is used, and its dry substancecontent can be high, the dryer output required is significantly lessthan in the manufacture of LWC, for example. Thanks to the reduction indryer output, the machine can be shortened and the energy consumptionper tonne of paper manufactured reduced.

If multi-nip calendering takes place before coating, an extremely smoothpaper surface is obtained before coating and the same smoothness istransferred to the properties of the coated paper. The calenderedsurface absorbs less water and the processed fibres expand less. Thus,thanks to precalendering, there is no substantial fibre rougheningduring coating, which would increase the roughness of the surface aftercoating. This property is extremely important when using a thin coating,because a thin coating layer will not cover possible roughening as well.In the same way, the small amount of water contained in a very thincoating layer will reduce the absorption of water by the fibres,accelerating drying and shortening the absorption time of the water.

While the invention is preferably applied in on-line processes, makingthe entire manufacturing line compact, it is also possible to apply theinvention to solutions, in which the manufacturing of the base paper,calendering, and coating take place in different stages. However, insuch cases, it may be difficult to control aspects such as the moisturecontent of the paper going to calendering, without additional wetting,which will increase costs. In other ways too, off-line manufacturingprocedures are usually more expensive than on-line methods.

Due to the small amount of coating, little wetting of the base paper isneeded, so that runnability remains good, as the wetting does notdecrease the strength of the base paper. Also the paper fibres onlyexpand slightly, partly due to the effect of calendering and partly dueto the small amount of water absorbed.

Other objects and features of the present invention will become apparentfrom the following detailed description considered in conjunction withthe accompanying drawings. It is to be understood, however, that thedrawings are intended solely for purposes of illustration and not as adefinition of the limits of the invention, for which reference should bemade to the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention is examined with the aid of theaccompanying drawing, which shows one arrangement for manufacturingpaper according to the invention.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

The FIGURE shows a paper machine finishing section, which is arrangedaccording to the invention. In this case, the paper is made in a papermachine and dried to the desired final dryness in the dryer cylindergroup 1. From dryer cylinder group 1, the paper web is taken to amulti-nip calender 3, which is preferably an OptiLoad calender, in whichthe pressures in the various roll nips can be adjusted independently,unlike in a super-calender, in which the nip load is created bycompressing the entire group of rolls. In addition to an OptiLoad and asuper-calender, other multi-nip calenders that achieve a sufficient websmoothness, can be used, for example, Janus Concept calenders. AnOptiLoad calender consists of hard heated rolls 4 arranged on top ofeach other, with soft rolls 5 set between them. The arrangement of therolls can be varied, and both the soft and hard rolls can be used asloading rolls, according to the construction of the calender. The paperweb 2 is guided through the calender and roll nips by means of guiderolls 6. Because multi-nip calenders are, as such, known, theirconstruction and the various loading and roll-nip formations and methodsof heating the rolls are not described here in greater detail.

After calender 3, the web is led to coating station 7, which, in thisexample, is a two-sided, film-transfer coater. Alternatively, thecoating station may be a two-sided spray coating station, or twoone-sided stations set sequentially. The use of various kinds of shortdwell time coaters can also be considered, but it is very difficult touse them to achieve the small amounts of coating in the preferredoperating range of the invention. The use of blade coaters, foamcoaters, or curtain coaters can also be considered, but the filmtransfer method and spray application appear to be preferable in termsof paper quality and can be used to achieve the most advantageous amountof coating from the point of view of the invention. After the coatingstation, there is a contactless dryer 8, which can be of any known type,and a dryer cylinder group 9. From dryer cylinder group 9, the web istaken to winder 11 through drive nips 10, which help to maintain thetension of the web. If it is desired to further improve the glosscontrast of the paper, the paper can also be treated, for example, witha soft calender, before winding. In place of the manner of dryingreferred to above, other drying methods and combinations of them canalso be used.

According to the invention, the paper 2 manufactured in the papermachine is dried to a suitable moisture content for calendering and thenled to multi-nip calender 3. The moisture content of paper to becalendered should be 4-14% of the total weight of the paper, the typicalmoisture percentage being 7-10%, preferably 8-10%. At this papermoisture content, a multi-nip calender can be used to achieve a PPS-S10roughness, which is at most 2.5 μm and even 1-1.5 μm PPS-S10.Calendering also compacts the surface of the paper. In such a case, thequality of the paper surface is optimal for film-transfer coating andalso highly suitable for spray coating. A common feature of thesecoating methods is that they can spread a very small amount of coatingevenly on the surface of the paper. Because the surface of papercalendered in the manner described above is extremely dense and even,very little coating containing dry substances penetrates between thefibres of the paper and there are no unevennesses to create valleys inwhich the coating could collect. Thus, the excellent evenness of thepaper becomes repeated as evenness in the coated surface and the surfaceacquires an excellent quality. Even a thin layer of coating spread on aneven surface will give a good cover, because the coating layer is of aneven thickness over the whole surface of the paper, so that its coveringpower is exploited optimally. The coating used can be any so-calledpigment coating, such as kaolin or calcium carbonate, which containssolid particles, polymer compounds, or similar, which remain on thesurface of the paper. According to the preferred embodiment of theinvention, the base paper contains a considerable amount of filler, i.e.at least 15% and preferably at least 20%. The filler is used to improvethe calendering result and can reduce the price of the base paper evenfurther. The filler used can be any filler normally used in papermaking, for example, the same substances that are used in the coating.

The method according to the invention has been used in trial runs, inwhich coating amounts of 2, 3, and 5 g/m²/side were spread onto thesurface of super-calendered paper, using a spray coater and an Optisizerfilm-transfer coater. The best quality was achieved using an amount of2-3 g/m² to coat the surface of super-calendered paper. The quality ofthe printed surface was better than that of the reference,super-calendered filler containing paper of 57 g/m² SC, the fibreroughness of the paper being better than that of 51 g/m² LWC. Anexcellent gloss contrast was achieved between the printed and unprintedsurfaces, without final calendering,.

EXAMPLE

The following example compares the advantages of the method according tothe invention with a product of the traditional LWC manufacturingmethod, which has a grammage of 60 g/m².

Aspect compared New concept LWC Mechanical pulp 40 g/m² 22 g/m² Basepaper chemical pulp — 16″ Base paper ash 16″  2″ Coating amount  4″ 20″Moisture content after paper machine 8-10% 2-3% Efficiency 88%  82%Coating 2 side SS 2 blade Coating drying 1 × 4 g/m² 2 × 10 g/m² Machinelength 85% 100%

The mechanical pulp and base paper pulp above refer to the fibrematerial used in the base paper, the base paper ash refers to the amountof ash remaining after combustion, which particularly depicts the amountof filler. The efficiency is the operating efficiency, which depicts thenumber of breaks. Coating was carried out on two Symsizer™ film-transfercoaters and correspondingly on two blade coaters. As the comparisonshows, the arrangement according to the invention is superior in termsof the drying arrangements, if the amount of energy required and thelength of the machine are taken into account. The length of the machinecorrelates directly with its price.

The following prices (in Finnish Marks) can be used to calculate therelative manufacturing costs for paper according to the invention andthe LWC paper referred to above.

Mechanical pulp FIM 1500/t Chemical pulp FIM 2500/t Ash FIM 600/tCoating FIM 1500/t Energy FIM 100/MWh

Because the prices of the various production factors may vary, theprices given above are only indicative. A calculation based on themshows that its is about 28%/tonne cheaper to manufacture paper accordingto the invention than to manufacture LWC paper. The greatest savings aredue to the possibility to use paper made from groundwood, instead ofpaper containing chemical pulp. The rest of the savings arise fromreplacing fiber with filler and from energy savings. In addition,improved efficiency increases production by 6% and reduces investmentcosts.

Embodiments of the invention, differing from those disclosed above, canalso be envisaged.

Naturally, the base paper of the paper according to the invention cancontain or be woodfree pulp, but this will increase its pricesubstantially. Measurement and control devices to control the processare an essential part of the equipment. These are, for example, moisturecontent measurement and control devices. However, these devices do notcome within the scope of the invention and are not described in greaterdetail.

Thus, while there have been shown and described and pointed outfundamental novel features of the present invention as applied to apreferred embodiment thereof, it will be understood that variousomissions and substitutions and changes in the form and details of thedevices described and illustrated, and in their operation, and of themethods described may be made by those skilled in the art withoutdeparting from the spirit of the present invention. For example, it isexpressly intended that all combinations of those elements and/or methodsteps which perform substantially the same function in substantially thesame way to achieve the same results are within the scope of theinvention. Substitutions of elements from one described embodiment toanother are also fully intended and contemplated. It is also to beunderstood that the drawings are not necessarily drawn to scale but thatthey are merely conceptual in nature. It is the intention, therefore, tobe limited only as indicated by the scope of the claims appended hereto.

What is claimed is:
 1. A method for producing a coated and calenderedpaper, comprising: adjusting the moisture content percentage of amanufactured base paper to between 4 and 14% of the total weight of thepaper; bringing the base paper web with said moisture content to amulti-nip calender; calendering the base paper in the multi-nip calenderto give it a maximum PPS-S10 roughness of 2.5 μm; and coating thecalendered base paper with at least one layer of coating.
 2. The methodof claim 1 wherein the base paper contains at least 15% of filler. 3.The method of claim 1 wherein the base paper contains at least 20% offiller.
 4. The method of claim 1 wherein the calendered base paper iscoated with a layer of coating with a maximum grammage of 5 g/m².
 5. Themethod of claim 2 wherein the calendered base paper is coated with alayer of coating with a maximum grammage of 5 g/m².
 6. The method ofclaim 3 wherein the calendered base paper is coated with a layer ofcoating with a maximum grammage of 5 g/m².
 7. The method of claim 1,wherein the calendered base paper is coated with a layer of coating witha maximum grammage of 3 g/m².
 8. The method of claim 2, wherein thecalendered base paper is coated with a layer of coating with a maximumgrammage of 3 g/m².
 9. The method of claim 3 wherein the calendered basepaper is coated with a layer of coating with a maximum grammage of 3g/m².
 10. The method of claim 1, wherein the base paper is calendered togive it a PPS-S10 roughness of 1 to 1.5 μm.
 11. The method of claim 2,wherein the base paper is calendered to give it a PPS-S10 roughness of 1to 1.5 μm.
 12. The method of claim 3, wherein the base paper iscalendered to give it a PPS-S10 roughness of 1 to 1.5 μm.
 13. The methodof claim 1, wherein both sides of the paper are coated simultaneously.14. The method of claim 4, wherein both sides of the paper are coatedsimultaneously.
 15. The method of claim 5, wherein both sides of thepaper are coated simultaneously.
 16. The method of claim 6, wherein bothsides of the paper are coated simultaneously.
 17. The method of claim 1,wherein the base paper is coated by film-transfer coating.
 18. Themethod of claim 2, wherein the base paper is coated by film-transferscoating.
 19. The method of claim 4, wherein the base paper is coated byfilm-transfer coating.
 20. The method of claim 5, wherein the base paperis coated by film-transfer coating.
 21. The method of claim 1, whereinthe base paper is coated by spray-application coating.
 22. The method ofclaim 2, wherein the base paper is coated by spray-application coating.23. The method of claim 4, wherein the base paper is coated byspray-application coating.
 24. The method of claim 5, wherein the basepaper is coated by spray-application coating.
 25. The method of claim 1,wherein the paper is calendered using a calender comprising a pluralityof hard heated rolls arranged on top of each other and a plurality ofsoft rolls set in between the hard heated rolls.
 26. The method of claim1, wherein the moisture content of the base paper is adjusted in a dryercylinder group, and the base paper web is taken directly from the dryercylinder group to the calender and from the calender directly to acoating station.
 27. Paper comprising a base paper calendered in amulti-nip calender to a maximum PPS-S10 roughness of 2.5 μm and, atleast on one side of the paper, a layer of coating with a maximumgrammage of 5 g/m².
 28. The paper of claim 27, wherein the grammage ofat least one layer of coating is a maximum of 3 g/m².
 29. The paper ofclaim 27, wherein the base paper contains mechanical pulp.
 30. The paperof claim 27, in which the base paper contains at least 15% of filler.31. The paper of claim 27, wherein the base paper comprises groundwoodand filler.
 32. The paper of claim 29, wherein the base paper comprisesgroundwood and filler.
 33. The paper of claim 30, wherein the base papercomprises groundwood and filler.
 34. The paper of claim 27, wherein thebase paper's PPS-S10 roughness is a between 1 and 1.5 μm.
 35. The paperof claim 30, wherein the base paper's PPS-S10 roughness is a between 1and 1.5 μm.
 36. The paper of claim 30, wherein the base paper containsat least 20% of filler.
 37. The paper of claim 31, wherein the basepaper contains at least 20% of filler.
 38. The paper of claim 32,wherein the base paper contains at least 20% of filler.
 39. The paper ofclaim 33, wherein the base paper contains at least 20% of filler.